Start using mock

I wasn't too satisfied with the current way to mock objects, so I started researching about it. The [Mock library](http://www.voidspace.org.uk/python/mock/) is actually part of the standard library in Python >= 3.3; so, looking to the future, it won't be another testing dependency (which is best to keep to a minimum).

I want some feedback on this, but I think this is the way.